In hierarchical computer systems, protection checking must occur whenever a process having lower privilege desires to call on a system wide service routine having higher privilege. Several levels of privilege are usually provided in such systems including a highest privilege available only to the operating system, one or more intermediate privilege levels available to either the operating system or intermediate program supervisors, and a lowest privilege level available to unprivileged operations and to user programs. When a user program needs to call the computer operating system to do something which can only be accomplished by the operating system itself, such as access a section of the physical memory in a virtually addressed computer memory, some form of protection must be provided so that the user program can only use the highly privileged routine in an orderly way. Without some form of protection there would be no data security in the system and any program would have unlimited access to all of the system data or a user program could actually destroy the operating system itself, both of which are unacceptable in most commercial systems. For example, if a payroll file is stored on the computer system and the system lacked privilege protection, someone could merely write a user program to give himself a raise.
Such protection checking is a frequent operation that usually involves significant system resources in validating the privileges and access rights of the calling routine, resulting in a substantial degradation in system performance. Previous systems have frequently used "supervisor call" instructions to limit the number of entry points available into the operating system. With such supervisor call instructions, a user program seeking higher privilege actually causes a hardware interrupt of the system processor, during which the system will branch to a single operating system location. The operating system must then figure out what it was that the user wanted to do, branch out through a large privilege dispatch table, perform the desired operation, return to the single operating system location, store all the desired system states, and return to the user program. Not only is such a supervisor call time consuming, but such a system requires the supervisor call to be a unique instruction which is different from other normal procedure calls.